Fuel cells, particularly solid polymer fuel cells, have advantages in that they operate at low temperatures and are compact, and thus are expected to serve as a power supply for home use and automobiles. A solid polymer fuel cell has a laminate structure including a hydrogen electrode (negative electrode), an air electrode (positive electrode), and a solid (polymer) electrolyte sandwiched between these electrodes. Then, a hydrogen-containing fuel is supplied to the hydrogen electrode, while oxygen or air is supplied to the air electrode, and electric power is extracted by utilization of the oxidation and reduction reactions at the respective electrodes.
Such reactions at the electrodes of a fuel cell are known to proceed at the point of contact (three-phase interface) of a reaction gas, a catalyst (electronic conductor), and a solid electrolyte (proton conductor), and it was believed that the presence of a solid electrolyte as well as a catalyst is indispensable. However, solid electrolytes have problems in that they may degrade due to radical species resulting from the electrode reactions, and also that they are relatively expensive and affect the electrode cost. Thus, the reduction of the amount of solid electrolyte added is demanded.
In a method proposed as a measure for solid electrolyte reduction, proton conductivity is imparted to a catalyst itself, which is an electronic conductor. Specifically, a catalyst for a fuel cell generally having a structure in which precious metal fine particles are supported on a carrier, the catalyst having sulfo groups (—SO3H) introduced into the carrier to impart proton conductivity, is developed. Patent Document 1 discloses a carrier having sulfo groups introduced into a condensed hydrocarbon bearing two or more aromatic rings, and Patent Document 2 discloses a catalyst provided with a sulfated zirconia carrier having sulfo groups supported on the zirconia surface.